Según yo la forma correcta de decirlo es:
A Mateo y Marta les duelen los ojos.
It means the u can use it in sentences
Answer:
So this translates to: Do you sometimes go to the library during the week?
So, the word is most likely: Cuando.
So this translates to:
<u><em>When do you sometimes go to the library during the week.</em></u>
Just my opinion and how I see it, feel free to use it if needed.
Explanation: